PCB calculators / Layout & fabrication
Free tool · PreBuild Engineering
PCB impedance calculator
Get close to 50 Ω single-ended or 90/100 Ω differential before you ask your fab for a stack-up.
Inputs
mm
mm
mm
Result
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Why it matters
USB, Ethernet, HDMI, MIPI and RF lines need controlled impedance. A mismatch causes reflections, eye closure and EMI. Early estimates let you pick a stack-up before layout, instead of rerouting afterwards.
Formula
Microstrip: Hammerstad–Jensen model with thickness correction
Differential microstrip ≈ 2·Z₀·(1 − 0.48·e^(−0.96·s/h))
Stripline (IPC-2141): Z₀ = 60/√εr · ln(4b / (0.67π(0.8w + t)))
Differential stripline ≈ 2·Z₀·(1 − 0.347·e^(−2.9·s/b))
Design tips
- FR-4 Dk varies from about 3.8 to 4.6 with glass style and frequency: use your fab's value.
- Keep a solid reference plane under every controlled-impedance trace; gaps and splits break the impedance.
- USB 2.0 needs 90 Ω differential; Ethernet and most LVDS need 100 Ω.
Results are engineering estimates from standard formulas and published approximations. Always confirm against component datasheets, your fabricator's capabilities and, for critical designs, simulation or measurement.